Membrane Structure and Function: Study Notes (Campbell Biology, Chapter 7)

Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all living cells, responsible for regulating the movement of substances into and out of the cell. Its unique composition and organization allow for selective permeability, communication, and compartmentalization essential for cellular function.

Major Ways the Plasma Membrane Regulates Inbound and Outbound Traffic

• Passive Transport: Movement of small molecules across the membrane without energy input, either by diffusion or via transport proteins.

• Active Transport: Movement of small molecules against their concentration gradient, requiring energy (usually ATP) and a transport protein.

• Bulk Transport (Exocytosis and Endocytosis): Movement of large molecules (such as proteins and polysaccharides) via vesicles, involving membrane fusion or invagination.

Membrane Structure: The Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, carbohydrates, and other lipids. This model explains both the fluidity and the diversity of membrane components.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer that serves as the basic framework of the membrane.

• Proteins: Integral and peripheral proteins are interspersed throughout the bilayer, contributing to membrane functions such as transport, signaling, and structural support.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), functioning in cell recognition and signaling.

Additional info: The term amphipathic refers to molecules that have both hydrophilic and hydrophobic regions, a property crucial for membrane formation.

Membrane Fluidity

Membrane fluidity is essential for proper function, affecting the movement of proteins and lipids within the bilayer and the ability of the membrane to change shape.

• Saturated Fatty Acids: Pack tightly, making the membrane more viscous and less fluid, especially at lower temperatures.

• Unsaturated Fatty Acids: Have kinks due to double bonds, preventing tight packing and increasing fluidity, especially beneficial at lower temperatures.

• Cholesterol: Acts as a fluidity buffer in animal cell membranes. At moderate temperatures, it reduces fluidity by restraining phospholipid movement; at low temperatures, it prevents solidification by disrupting packing.

Types of Membrane Proteins

Membrane proteins are critical for the diverse functions of the plasma membrane.

• Integral Proteins: Penetrate the hydrophobic core of the bilayer; many are transmembrane proteins that span the membrane.

• Peripheral Proteins: Bound to the surface of the membrane, often attached to integral proteins or the cytoskeleton.

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Role of Membrane Carbohydrates

Carbohydrates on the cell surface are involved in cell recognition and communication.

• Glycoproteins: Carbohydrates attached to proteins.

• Glycolipids: Carbohydrates attached to lipids.

• Function: Serve as markers for cellular identification and play a role in immune response.

Selective Permeability of the Membrane

The plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

• Hydrophobic (nonpolar) molecules: Pass through the lipid bilayer rapidly (e.g., O2, CO2).

• Hydrophilic (polar) molecules: Pass slowly or require transport proteins (e.g., glucose, ions).

Transport Proteins

Transport proteins facilitate the movement of specific substances across the membrane.

• Channel Proteins: Provide hydrophilic tunnels for molecules or ions to pass through (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane; highly specific for their substrate.

Passive Transport: Diffusion and Osmosis

Passive transport is the movement of substances down their concentration gradient without energy input.

• Diffusion: Movement of molecules from high to low concentration until equilibrium is reached.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmosis Equation:

$ext{Water potential} ( ext{Ψ}) = ext{solute potential} ( ext{Ψ}_s) + ext{pressure potential} ( ext{Ψ}_p)$

Effects of Tonicity on Cells

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

Environment	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Additional info: Cells without walls (e.g., animal cells) are sensitive to tonicity, while plant cells rely on turgor pressure for structural support.

Facilitated Diffusion

Facilitated diffusion is passive transport aided by proteins, allowing specific molecules to cross the membrane more efficiently.

• Channel Proteins: May be gated, opening in response to stimuli.

• Carrier Proteins: Undergo conformational changes to move substances down their concentration gradient.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually from ATP).

• Sodium-Potassium Pump: Maintains high K+ and low Na+ concentrations inside animal cells.

Equation for Sodium-Potassium Pump:

$ext{3 Na}^+ ext{out} / ext{2 K}^+ ext{in} / ext{per ATP hydrolyzed}$

Membrane Potential and Electrochemical Gradients

Membrane potential is the voltage across a cell membrane, resulting from the unequal distribution of ions.

• Electrogenic Pumps: Transport proteins that generate voltage across the membrane (e.g., sodium-potassium pump in animals, proton pump in plants).

• Electrochemical Gradient: Combination of chemical and electrical forces driving ion movement.

Bulk Transport: Exocytosis and Endocytosis

Bulk transport moves large molecules and particles across the membrane via vesicles.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., secretion of insulin).

• Endocytosis: The plasma membrane engulfs material, forming a vesicle to bring substances into the cell.

• Types of Endocytosis:

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid and dissolved solutes.

  • Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor binding.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Passive Transport	No	Down gradient	O2 diffusion
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Bulk Transport	Yes (ATP)	In or out	Exocytosis of insulin

Examples and Applications

• Paramecium: Uses contractile vacuole for osmoregulation in hypotonic environments.

• Plant Cells: Rely on turgor pressure for structural integrity.

• Human Cells: Use receptor-mediated endocytosis to import cholesterol via LDL particles.